feat(chrome): shared account menu and global app header

Move account actions (Settings, Downloads, Display preferences, Sign
out) out of the library-only header into a shared AccountMenu anchored in
a global AppHeader, available on every authenticated non-immersive
screen. Add a layoutShell helper deciding where chrome shows, expose
serverName/serverUrl auth stores, and a display view-mode preference. The
settings page also gains the UR-053 WiFi-only toggle.

TRACES: UR-054 | DR-075, DR-076, DR-077

src/lib/utils/layoutShell.ts

@@ -67,6 +67,29 @@ export function routeOwnsLayout({ pathname }: { pathname: string }): boolean {
   );
 }
 
+/**
+ * Whether the root layout should render the shared app header (logo, desktop
+ * nav, and the account menu) for this route.
+ *
+ * Routes that own their layout (library) render their own AppHeader, so the
+ * root must not double it up. `/settings` owns its content but deliberately has
+ * no account menu (the user is already there). `/player/*` and `/login` are
+ * immersive/chrome-free. Everything else authenticated (`/`, `/search`,
+ * `/downloads`) gets the header from the root — the whole point of UR-054.
+ *
+ * TRACES: UR-054 | DR-076
+ */
+export function showGlobalHeader({
+  pathname,
+  isAuthenticated,
+}: BottomUiVisibilityInput): boolean {
+  return (
+    isAuthenticated &&
+    !routeOwnsLayout({ pathname }) &&
+    !pathname.startsWith("/settings")
+  );
+}
+
 /**
  * Whether any bottom UI is showing for this route (mini player, nav, or both).
  * The bottom UI is rendered in flex flow below the scroller (see BottomUi.svelte),